Method and apparatus for automatic and autonomous assignment of PRN codes to a multiplicity of pseudolites

ABSTRACT

A pseudolite for transmitting a global positioning system (GPS) signal having identification information for a GPS satellite that is currently out-of-view. The pseudolite mimics a global positioning system (GPS) signal by using a C/A pseudorandom (PRN) identification code for a GPS satellite that is allocated in the GPS system but is unreceivable in the local vicinity and location-determination information including ephemeris data corresponding to the geographical location of the pseudolite. In order to ensure that two of the GPS pseudolites in the same vicinity do not use the identification PRN code from the same out-of-view GPS satellite, the pseudolite listens first before transmitting to detect the identifications in the received GPS signals.

This is a continuation of application Ser. No. 09/409,023 filed Sep. 29,1999, now U.S. Pat. No. 6,198,432, issued Mar. 6, 2001. The priority ofthat application is claimed under 35 U.S.C. § 120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to pseudolites for satellite positioningsystems and more particularly to a pseudolite for transmitting apositioning system signal having identification informationcorresponding to a positioning system satellite that is currentlyout-of-view.

2. Description of the Prior Art

The global positioning system (GPS) operated by the United Statesgovernment uses microwave transmissions from orbiting satellites withknown orbits. These transmissions are received by a satellite navigationreceiver for determining the location of the receiver. Such microwavefrequencies are blocked by the earth or by local obstructions such asbuildings. Locating the receiver in an urban city block or inside abuilding can severely limit performance by excluding necessarysatellites from a constellation being tracked.

Each of the GPS satellites transmits a GPS signal on the same carrierfrequency modulated by location-determination information from that GPSsatellite and spread by pseudorandom (PRN) codes that are distinct forthat GPS satellite. Two different PRN codes are used by each satellite:a long code termed the precise/encrypted (P/Y) code and a short code of1023 bits or chips termed the coarse/acquisition (C/A) code. Either theP/Y code or the C/A code identifies the GPS satellite transmitting theGPS signal and enables a GPS navigation receiver to distinguish the GPSsignal from one GPS satellite from the GPS signal from another GPSsatellite. The P/Y code is encrypted and restricted for use to thoseauthorized by the United States Department of Defense while knowledge ofthe C/A code is available to all users.

There are over one thousand distinct C/A PRN codes that could be usedfor identification for distinguishing the GPS satellites. Of these C/APRN codes, the United States government has currently allocated aboutthirty-two for use by GPS satellites. Existing GPS receivers aredesigned to search for GPS signals from GPS satellites having any one ofthese codes. Of these thirty-two allocated satellites, currently abouttwenty-seven are orbiting and operational and five are not operational.Typically, of the orbiting operational satellites, in mid-latitudesabout one-third will be above the Earth horizon and potentiallyreceivable by a GPS receiver and about two-thirds will be unreceivableto the receiver by being below the horizon. Therefore, abouttwenty-three of the satellites will not be receivable for use by the GPSreceiver.

Existing GPS applications use pseudolites to augment the satelliteconstellation and thus improve availability of the GPS signal. Suchpseudolites mimic the satellite transmissions by broadcasting pseudo GPSsignals, but are fixed on the ground and transmit thelocation-determination information appropriate to the geographicallocation of the pseudolite. The pseudolites make use of PRN codes thathave not been allocated for GPS satellites. Signal reception is nearlyguaranteed when the pseudolite is located nearby due to relativelyhigher signal strength of the received pseudo GPS signal. In addition tothe thirty-two PRN codes allocated for GPS satellites the United Statesgovernment currently allocates about four codes for the use ofpseudolites. For example, a pair of pseudolites at the end of anairport's runway are conventionally used to enhance the positiondetermination of a navigational receiver in a landing aircraft. It hasbeen proposed that several pseudolites be used in a metropolitan area inorder to improve GPS service in urban canyons and inside of buildings.

Fast GPS signal acquisition is important in applications for many GPSnavigation receivers. For example, a battery powered receiveralternating between operational and standby modes needs a fastacquisition in order to have good battery life with small batteries. Onetechnique for achieving a fast acquisition is to minimize the number ofPRN codes or other types of identifications that are searched in orderto acquire GPS signals. However, a requirement for more than fourpseudolites increases the number PRN codes that must be stored orgenerated in a GPS receiver, thereby slowing signal acquisition time incertain circumstances.

There is a need for a pseudolite using a PRN code identification thatminimizes the number of PRN codes that must be stored for search in aGPS receiver. Further, there is a need for an autonomous assignmentprocedure that simplifies and automates the choosing process, especiallywhen there are a multiplicity of pseudolites to be installed, and thepseudolites could be supplied by different manufacturers.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide apositioning system (PS) pseudolite for a satellite positioning systemwhere the PS pseudolite transmits a pseudo PS signal using a PSsatellite identification for a PS satellite that is allocated for thepositioning system but is currently unreceivable in the vicinity of thePS pseudolite.

The PS pseudolite of the present invention operates autonomously andindependently from any central authority for determining whichparticular one of the unreceivable PS satellite identifications is usedfor transmitting a pseudo PS signal.

Briefly, in a preferred embodiment, a positioning system (PS) pseudoliteof the present invention is a global positioning system (GPS)pseudolite. The GPS pseudolite mimics a global positioning system (GPS)signal by using a C/A pseudorandom (PRN) identification code for a GPSsatellite that is allocated in the GPS system but is unreceivable in thelocal vicinity because it is below the local horizon in its orbit. Thepseudolite transmits location-determination information includingephemeris data corresponding to the geographical location of thepseudolite. In order to ensure that two of the GPS pseudolites in thesame vicinity do not use the identification PRN code from the sameunreceivable GPS satellite, each GPS pseudolite listens first beforetransmitting to detect the identifications in the received GPS signalsand uses PRN codes that are not being received. This autonomous codeassignment process is independent of any central authority so thatmultiple vendors can supply pseudolites even in the same local area.

The GPS pseudolite includes a satellite availability calculatorincluding a visibility calculator and an operational identifier. Thevisibility calculator uses GPS satellite almanac and/or ephemerisorbital parameter data for determining the GPS satellites that arein-view having a line-of-sight to the GPS pseudolite and the GPSsatellites that are out-of-view, for example behind the Earth. Theoperational identifier determines the GPS satellites that areoperational and those that are non-operational. Non-operational GPSsatellites include those GPS satellites that have not been launched orhave been turned off. The GPS satellites that are both operational andin-view are designated as receivable GPS satellites. The GPS satellitesthat are either out-of-view or non-operational are designated asunreceivable GPS satellites. The GPS pseudolite further includes a GPSreceiver, a pseudolite detector, a satellite identification selector,and a pseudolite GPS transmitter. The GPS receiver receives GPS signalsfrom the receivable GPS satellites and pseudo GPS signals from other GPSpseudolites in the vicinity and passes the received identifications tothe pseudolite detector. The pseudolite detector detects that anotherGPS pseudolite in the vicinity is transmitting when a receivedidentification matches the identification of an unreceivable GPSsatellite. The satellite identification selector selects an availableidentification that both corresponds to one of the unreceivable GPSsatellites and is not currently being used by another GPS pseudolite.The pseudolite transmitter then transmits a pseudo GPS signal having theselected available PRN code identification.

An advantage of the PS pseudolite of the present invention for apositioning system is that the PS pseudolite re-uses identificationsthat have been allocated for positioning system satellites, thusreducing the search time for a remote GPS receiver to acquire apseudolite transmission.

Another advantage of the PS pseudolite of the present invention is thatno coordination is needed by either manufacturers or system integratorswith regard to selection of unique and independent identifications.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodimentswhich are illustrated in the various figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a global positioning system (GPS) having a GPS pseudolite of the present invention;

FIG. 2 is a block diagram of the GPS pseudolite of the globalpositioning system of FIG. 1; and

FIG. 3 is a table of allocated GPS satellites for the global positioningsystem of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an autonomous code assignment pseudolite system ofthe present invention referred to by the general reference number 10.The system 10 includes a global positioning system (GPS) navigationreceiver 12 on or near the Earth 14 for receiving GPS signals fromin-view GPS satellites 16 having a line-of-sight to the receiver 12 andpseudo GPS signals from one or more nearby GPS pseudolites 20 of thepresent invention. The GPS navigation receiver 12 uses the GPS signalsand pseudo GPS signals for determining its geographical location andtime. Although only one GPS navigation receiver 12 is shown in FIG. 1,it is envisioned that the system 10 may include several GPS navigationreceivers 12. Of course, the GPS navigation receivers 12 may be used forapplication that are not strictly navigation, such as surveying,mapping, event tagging, and the like where-location, velocity and/ortime is required.

Out-of-view GPS satellites 24 not having a line-of-sight to the GPSnavigation receiver 12 are typically not receivable by the receiver 12because their GPS signals are blocked by the Earth 14. The in-view GPSsatellites 16 orbit the Earth 14 in a period of approximately twelvehours so that each of the in-view GPS satellites 16 periodically setsbelow the horizon of the Earth 14 and becomes an out-of-view GPSsatellite 24, then rises to become an in-view GPS satellite 16, and soon. Similarly, each of the out-of-view GPS satellites 24 periodicallyrises above the horizon of the Earth 14 and becomes an in-view GPSsatellite 16, then sets to become an out-of-view GPS satellite 24, andso on.

The GPS signal is formatted according to GPS specifications for carryingsatellite identification information and location-determinationinformation. A written source of such GPS specifications is availablefrom ARINC Research Corporation of El Segundo, Calif. under the title of“GPS Interface Control Document ICD-GPS-200, NAVSTAR GPS Space Segmentand Navigation User Interfaces” revised in Sep. 25, 1997.

Briefly, the GPS signal has an L-band carrier signal modulated by GPSdata bits of twenty milliseconds that are spread by a pseudorandom (PRN)code that repeats every one millisecond. The GPS data bits and the PRNcodes of all the GPS signals are synchronized to transmit at the sametimes beginning with 00 hours, 00 minutes, 00.000 seconds of each GPSweek and continuing throughout the week. The PRN code in each GPS signalis distinct, thereby allowing a GPS receiver to use the PRN code asidentification information for distinguishing the GPS signal from oneGPS signal source from the GPS signal from another GPS signal source.The United States government has allocated about thirty-two of these PRNcodes for current use in the global positioning system. However, aboutfive GPS satellites are currently non-operational, and therefore,unreceivable because they have not been launched or have beendecommissioned. The particular PRN codes that are operational variesover time as old GPS satellites are retired and new ones are launched.

The GPS data bits carry location-determination information segmentedinto 1500 bit frames, also called pages, of thirty seconds. The framesare segmented into five 300 bit subframes of six seconds each. Thesubframes are segmented into thirty 10 bit words. Each subframe beginswith a known preamble and includes a Z-count. The Z-count givesGPS-based time-of-transmission for the preamble. Subframes two and threethe GPS signal from the GPS satellites 16, 24 and the GPS pseudolite 20includes ephemeris data for the GPS satellite 16, 24 or the GPSpseudolite 20 transmitting that GPS signal. The ephemeris data is highlyaccurate and is updated about hourly.

The ephemeris data for the GPS satellites 16, 24 describes the currentorbital parameters for the motion of the GPS satellites 16, 24 throughspace. The GPS navigation receiver 12 uses the ephemeris data along withthe GPS based-time for determining the time-variable locations-in-spacefor the in-view GPS satellites 16 whose GPS signals are received andprocessed. The ephemeris data for the GPS pseudolite 20 uses the sameformat for describing a ground-fixed location of the GPS pseudolite 20that has been determined with a survey. In a preferred embodiment, theGPS navigation receiver 12 determines the fixed location of the GPSpseudolite 20 in a similar way as for the locations-in-space of thein-view GPS satellites 16. Alternatively, because the ephemeris datadescribes a fixed location, the GPS navigation receiver 12 can determinethe location of the GPS pseudolite 20 without first determining aGPS-based time. With the four or more of the locations-in-space for thein-view GPS satellites 16 and/or the locations of the GPS pseudolites20, the GPS navigation receiver 12 can determine its own geographicallocation. Fewer than four in-view GPS satellites 16 and/or GPSpseudolites 20 are required when other navigation information such as anaccurate time, inertial information, map matching, or altitude isavailable. More than four in-view GPS satellites 16 and/or GPSpseudolites 20 are useful for improving the accuracy of the location.

Although the preferred embodiment is described in terms of the globalpositioning system the elements and methods of the present invention areapplicable to other satellite positioning systems such as the globalorbiting navigational system (GLONASS). GLONASS satellites transmitGLONASS signals having the same pseudorandom code that are identified bydistinct carrier frequencies. A comparable GLONASS pseudolite systemwould use transmission frequencies based on those used by out-of-view ornon-operation GLONASS satellites.

FIG. 2 is a block diagram of the pseudolite of the present inventionreferred to by the general reference number 20. The pseudolite 20includes a pseudolite GPS receiver 42, a processor 44, a memory 46, apseudolite GPS transmitter 48, and an interface 52. The GPS receiver 42receives GPS signals from the in-view GPS satellites 16 (FIG. 1) and theother GPS pseudolites 20 (FIG. 1) and passes information to theprocessor 44 for GPS-based time, the GPS satellite almanac and/orephemeris orbital parameters, and the identifications of the satelliteand pseudo GPS signals that are being received. In general, theidentifications include the identifications corresponding to the in-viewGPS satellites 16 transmitted by the in-view GPS satellites 16, and theidentifications for the out-of-view GPS satellites 24 andnon-operational GPS satellites (FIG. 3) transmitted by the otherpseudolites 20. The processor 44, operates in a conventional manner forreading and writing data into the memory 46 and executing program codesin the memory 46 for receiving information and controlling the elementsof the GPS pseudolite 20 including the GPS receiver 42, the pseudolitetransmitter 48, and the interface 52. Preferably, the stored data in thememory 46 includes information for a fixed ground-based geographicallocation of the GPS pseudolite 20. Alternatively, the GPS receiver 42may be a survey grade GPS receiver equipped for real time operation forproviding and updating a precise geographical location for thepseudolite 20.

The memory 46 includes program codes for a satellite availabilitycalculator 62, a pseudolite detector 64, and a satellite identificationselector 66. The satellite availability calculator 62 includes avisibility calculator 74 and an operational identifier 76. Thevisibility calculator 74 uses the GPS satellite almanac or ephemerisorbital parameters for determining the in-view GPS satellites 16 havinga line-of-sight to the GPS pseudolite 20 and the out-of-view GPSsatellites 24 where the line-of-sight is blocked by the Earth 14 (FIG.1). The visibility calculator 74 can be programmed with the coordinatesand heights of local obstructions such as the walls of an open mine, abuilding, or a mountain range for differentiating in-view GPS satellites16 from out-of-view GPS satellites 24.

The operational identifier 76 uses the satellite orbital parameters andoptionally uses information received through the interface 52 that isprovided by the United States government for determining theidentifications for the orbiting operational GPS satellites 16, 24 andthe non-operational GPS satellites (FIG. 3). The satellite availabilitycalculator 62 designates the in-view GPS satellites 16 that areoperational as receivable GPS satellites. The out-of-view GPS satellites24 and the non-operational GPS satellites (FIG. 3) are designated asunreceivable GPS satellites. The pseudolite detector 64 detects anotherGPS pseudolite 20 in the vicinity when a received identification matchesthe identification of an unreceivable GPS satellite. The satelliteidentification selector 66 selects an available identification that bothcorresponds to one of the unreceivable GPS satellites and is notcurrently being used by another one of the GPS pseudolites 20.Preferably, the visibility calculator 74 calculates information forwhich of the out-of-view GPS satellites 24 have the longest time periodsbefore they become in-view GPS satellites 16 and provides thisinformation to the satellite identification selector 66. The satelliteidentification selector 66 then selects the identification for theout-of-view GPS satellite 24 having the longest time period that isavailable. The pseudolite GPS transmitter 48 then transmits a pseudo GPSsignal having the selected available PRN code identification andlocation-determination information for the geographical location of thepresent GPS pseudolite 20. The GPS navigation receiver 20 determines ageographical location from the GPS signals and the pseudo GPS signalsfrom the in-view GPS satellites 16 and the pseudolites 20, respectively.This method reduces the number of PRN codes that must be searched in theGPS receiver 12 to acquire a new pseudolite.

FIG. 3 is a table showing the PRN numbers 1-32 and the associatedorbital slots of the allocated GPS satellites for the global positioningsystem as of Sep. 29, 1999. The alpha identification character of theslot, A through F, represents the six orbits for the GPS satellites 16,24. The numeric identification character of the slot, 1 through 5,represents the five positions within the orbit. As of Sep. 29, 1999, PRNcodes 11, 12, 20, 28, and 32 are non-operational. A current status ofthe operational GPS satellites is available from the United Statesgovernment on-line at www.navcen.uscg.mil.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A pseudolite, comprising an availabilitycalculator using orbital parameters for positioning system (PS)satellites for identifying unreceivable ones of said PS satellites; anda transmitter for transmitting a local pseudo PS signal having aparticular one of several identifications corresponding to a particularone of said unreceivable PS satellites, wherein: the availabilitycalculator includes a visibility calculator for determining out-of-viewones of said PS satellites and identifying said out-of-view PSsatellites as said unreceivable PS satellites, wherein: said visibilitycalculator is for determining said out-of-view PS satellites based uponlocation information and height of at least one of a building, a wall,and a mountain range.
 2. The pseudolite of claim 1, wherein: theavailability calculator includes an operational identifier fordetermining non-operational ones of said PS satellites and identifyingsaid non-operational PS satellites as said unreceivable PS satellites.3. The pseudolite of claim 1, wherein: said PS satellites include GPSsatellites for transmitting GPS signals as said PS signals.
 4. Thepseudolite of claim 1, wherein: said PS satellites include globalorbiting navigation system (GLONASS) satellites for transmitting GLONASSsignals as said PS signals.
 5. A method in a pseudolite for augmenting apositioning system having positioning system (PS) satellites, comprisingsteps of: within said pseudolite, using orbital parameters of said PSsatellites for identifying unreceivable ones of said PS satellites; andtransmitting a local pseudo PS signal having a particular identificationcorresponding to a particular one of said unreceivable PS satellites,wherein: the step of identifying said unreceivable PS satellitesincludes steps of: determining out-of-view ones of said PS satellites;and identifying said out-of-view PS satellites as said unreceivable PSsatellites, wherein: said step of determining said out-of-view PSsatellites includes determining said out-of view PS satellites basedupon location information and height of at least one of a building, awall, and a mountain range.
 6. The method of claim 5, wherein: the stepof identifying said unreceivable PS satellites includes steps ofdetermining non-operational ones of said PS satellites; and identifyingsaid non-operational PS satellites as said unreceivable PS satellites.7. The method of claim 5, wherein: said PS satellites include GPSsatellites for transmitting GPS signals as said PS signals.
 8. Themethod of claim 5, wherein: said PS satellites include global orbitingnavigation system (GLONASS) satellites for transmitting GLONASS signalsas said PS signals.
 9. The method of claim 5, wherein: said local pseudoPS signal includes location-determination information for reception by aPS navigation receiver for determining at least one of (i) location and(ii) time.